Mixed cesium sodium and lithium halide scintillator compositions

ABSTRACT

The present invention relates to scintillator compositions and related devices and methods. The scintillator may include, for example, a mixed scintillator composition including at least two different CsXLa halide compounds and a dopant, wherein X is Na or Li. Related radiation detection devices and methods are further included.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. applicationSer. No. 11/938,176, entitled “Cesium and Lithium-Containing QuaternaryCompound Scintillators,” filed on Nov. 9, 2007 now U.S. Pat. No.7,655,919, and a continuation-in-part of U.S. application Ser. No.11/938,182, entitled “Cesium and Sodium-Containing ScintillatorCompositions,” filed on Nov. 9, 2007, and claims priority to U.S.Provisional Application No. 61/045,891, filed Apr. 17, 2008, the entirecontent of each of which is incorporated herein by reference.

The present application is related to U.S. Application No. 12,405,137,entitled “Cesium and Lithium-Containing Quaternary CompoundScintillators,” filed on Mar. 16, 2009 and U.S. application Ser. No.12/405,155, entitled “Cesium and Sodium-Containing ScintillatorCompositions,” filed on Mar. 16, 2009, each of which is being filedconcurrently herewith, the full disclosures of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to scintillator compositions and relateddevices and methods. More specifically, the present invention relates toscintillator compositions including a scintillation compound and adopant for use, for example, in radiation detection, including gamma-rayspectroscopy, and X-ray and neutron detection, imaging applications, andthe like.

Scintillation spectrometers are widely used in detection andspectroscopy of energetic photons (e.g., X-rays, gamma-rays, etc.). Suchdetectors are commonly used, for example, in nuclear and particlephysics research, medical imaging, diffraction, non destructive testing,nuclear treaty verification and safeguards, nuclear non-proliferationmonitoring, and geological exploration.

Important requirements for the scintillation crystals used in theseapplications include high light output, transparency to the light itproduces, high stopping efficiency, fast response, good proportionality,low cost, and availability in large volume. These requirements on thewhole cannot be met by many of the commercially available scintillatorcompositions. While general classes of chemical compositions may beidentified as potentially having some attractive scintillationcharacteristic(s), specific compositions/formulations having bothscintillation characteristics and physical properties necessary foractual use in scintillation spectrometers and various practicalapplications have proven difficult to predict. Specific scintillationproperties are not necessarily predictable from chemical compositionalone, and preparing effective scintillator compositions from evencandidate materials often proves difficult. For example, while thecomposition of sodium chloride had been known for many years, theinvention by Hofstadter of a high light-yield and conversion efficiencyscintillator from sodium iodide doped with thallium launched the era ofmodern radiation spectrometry. More than half a century later, thalliumdoped sodium iodide, in fact, still remains one of the most widely usedscintillator materials. Since the invention of NaI(Tl) scintillators inthe 1940's, for half a century radiation detection applications havedepended to a significant extent on this material. The fields of nuclearmedicine, radiation monitoring, and spectroscopy have grown up supportedby NaI(Tl). Although far from ideal, NaI(Tl) was relatively easy toproduce for a reasonable cost and in large volume. With the advent ofX-ray CT in the 1970's, a major commercial field emerged as did a needfor different scintillator compositions, as NaI(Tl) was not able to meetthe requirements of CT imaging. Later, the commercialization of positronemission tomography (PET) imaging provided the impetus for thedevelopment of yet another class of detector materials with propertiessuitable for PET. As the methodology of scintillator developmentevolved, new materials have been added, and yet, specific applicationsare still hampered by the lack of scintillators suitable for particularapplications.

As a result, there is continued interest in the search for newscintillator compositions and formulations with both the enhancedperformance and the physical characteristics needed for use in variousapplications. Today, the development of new scintillator compositionscontinues to be as much an art as a science, since the composition of agiven material does not necessarily determine its properties as ascintillator, which are strongly influenced by the history (e.g.,fabrication process) of the material as it is formed. While it may bepossible to reject a potential scintillator for a specific applicationbased solely on composition, it is typically difficult to predictwhether even a material with a promising composition can be used toproduce a useful scintillator with the desired properties.

One of the uses of radiation monitoring devices is preventing the spreadof weapons of mass destruction such as nuclear weapons. One way topassively determine the presence of nuclear weapons is to detect andidentify the characteristic signatures of special nuclear materials(SNMs) such as highly enriched uranium and weapons grade plutonium.Characteristic X-rays and gamma-rays are signatures of these materials.The general approach to a passive gamma-ray assay is to acquire rawspectra, correct the spectra for rate-related electronic losses andsource attenuation, and compute the total corrected count rate, which isproportional to the mass of the isotope being assayed. Theproportionality constant includes the effects of gamma-ray emissionrate, solid angle, and detector efficiency.

Monitoring for both highly enriched uranium and weapons grade plutoniuminvolves analysis of X-ray and gamma-ray spectra with multiple energiesof interest. One important consideration in SNM monitoring is thedetermination of uranium enrichment since highly enriched uranium can beused for development of nuclear weapons. The naturally occurringisotopic abundance of uranium is: ²³⁸U (99.27%), ²³⁵U (0.720%) and ²³⁴U(0.006%). When the fraction of the fissile ²³⁵U is higher than that innaturally occurring uranium, the uranium is said to be enriched. Therelative intensity of 185.7 keV gamma-rays (from ²³⁵U) compared to the94-98 keV X-rays for uranium in an unattenuated spectrum can be used todetermine uranium enrichment (Passive Nondestructive Assay of NuclearMaterials, eds. Reilly et al., U.S. Nuclear Regulatory Commission,Washington D.C., pp. 11-18, (1991)). As the uranium enrichment levelincreases, the relative intensity of the 185.7 keV gamma-ray peakincreases in comparison to the 94-98 keV X-ray peak. Anotherconsideration in SNM monitoring is to distinguish “weapons-grade”plutonium (with 93% ²³⁹Pu) from “reactor-grade” plutonium <60% ²³⁹Pu).The “reactor-grade” plutonium includes other isotopes such as ²⁴⁰Pu,²⁴¹Pu, ²⁴²Pu, and ²³⁸Pu. Comparison of gamma-ray signatures of ²³⁹Pu(such as 129.3 keV and 413.7 keV photons) with those for other plutoniumisotopes allows determination of the grade of plutonium. In addition tothe characteristic X- and gamma-rays of highly enriched uranium andweapons grade plutonium, there is considerable interest in themeasurement of irradiated fuel from nuclear reactors because of theplutonium produced during reactor operation. Due to very intensegamma-rays emitted by fission products of the irradiated fuel,gamma-rays from spontaneous decay of plutonium and uranium (²³⁵U, ²³⁹Puand ²⁴¹Pu) are generally not used for measurement of irradiated fuel.The most commonly measured fission product gamma-ray is the 662 keV linefrom ¹³⁷Cs. Recent threat of “dirty bombs” (devices which spreadradioactive material using conventional, non-nuclear explosives) hasalso created an interest in monitoring of radioactive materials such as¹³⁷Cs, ⁶⁰Co, ²⁴¹Am, radioactive medical waste, and irradiated fuel fromnuclear reactors that emit high energy gamma-rays.

Thus, gamma-ray spectrometers and radiation detectors are importanttools in monitoring of special nuclear materials. A number of homelandsecurity systems such as hand-held radioisotope identifiers, vehicleportals for radiation detection, and personal radiation detectiondevices rely on availability of high performance gamma-rayspectrometers. Similar systems are also required for nuclearnon-proliferation monitoring. An important challenge in homelandsecurity monitoring is not only to detect hidden radioactive materialsbut also to distinguish them from routinely used radiopharmaceuticals aswell as from naturally occurring benign radioactive materials.

Existing scintillator materials and commercial radiation detectors donot meet the current needs for radiation monitoring and weaponsdetection. For example, existing scintillator compositions and detectorstypically lack the one or more important scintillation properties (e.g.,high energy resolution, light output, stopping power, fast response, andthe like) that are desired and/or are not useful in detecting bothenergetic photons (e.g., gamma-rays and X-rays) as well as neutronemission. Thus, a need exists for improved scintillator compositionssuitable for use in various radiation detection applications, including,for example, radiation and nuclear weapons monitoring.

BRIEF SUMMARY OF THE INVENTION

The present invention provides scintillator compositions, includingmixtures of different quaternary scintillator compounds, and relateddevices and methods. In one embodiment, scintillator compositions of theinvention can, for example, include one or more compounds selected fromCs₂NaLaBr₆, Cs₂NaGdI₆, Cs₂NaLaI₆, and Cs₂NaLuI₆. These materials have incommon a quaternary composition comprising Cs and Na, a Lanthanide and aHalide. In another embodiment, a scintillator composition can includematerials that have in common a quaternary compound compositioncomprising Cs, Li, La and a Halide. Exemplary scintillator compositionscan, for example, include Cs₂LiLaF₆, Cs₂LiLaCl₆, Cs₂LiLaBr₆, andCs₂LiLaI₆. Scintillator compositions of the present invention caninclude a single type of scintillator compound or a mixture of differentcompounds (e.g., mixed scintillator compositions). Mixed scintillatorcompositions can include one or more of the cesium and sodium-containingquaternary compounds disclosed herein, or one or more of the cesium andlithium-containing quaternary compounds disclosed herein. In anotherembodiment, e.g., a composition can include a mixed composition thatincludes at least two different CsXLa halide compounds, where X is Na orLi. Exemplary halides can include F, Cl, Br, or I.

Excellent scintillation properties, including high light output, goodproportionality, fast response, and good energy resolution have beenmeasured for certain compositions of the present invention, which weredemonstrated to be suitable for gamma-ray spectroscopy, as well as X-rayand neutron emission detection. Scintillator compositions as describedherein are cubic (and therefore easy to grow) typically have high lightoutput and fast response. For gamma rays, the non-proportionality isparticularly outstanding and can translate into good energy resolution.These materials have very good non-proportionality (e.g., about 2%non-proportionality over 60 to 1000 keV energy range). The scintillatorcompositions show outstanding light output for neutrons in comparison topreviously existing neutron scintillators and can show pulse shapediscrimination between neutrons and gamma rays. Thus, these materialsadditionally advantageously combine excellent gamma-ray detectionproperties along with neutron detection.

Scintillation properties of the certain crystals/compositions of thepresent invention and described herein include peak emission wavelengthsfrom about 385 to about 475 nm, which is well matched to PMTs as well assilicon diodes used in nuclear instrumentation and a peak wavelength forgamma-ray spectroscopy. The principal decay-time constant in oneinstance was measured at approximately 50 ns, which is faster than thedecay-time constant of commercial scintillators such as BGO and GSO(see, e.g., Table 1 infra). Thus, in one embodiment, the presentinvention includes scintillator compositions having decay time constantsbetter than, or having a value of less than, 200 ns (e.g., less than 150ns). In some embodiments, the decay time constant can be better than 100ns, or even better than 50 ns. Under gamma ray excitation, the lightoutput typically will be greater than about 10,000 photons/MeV, and insome instances greater than about 20,000, 40,000, or 60,000 photons/MeV(e.g., ranging from about 17,000 to about 60,000 photons/MeV andgreater), which is greater than that of many widely used commercialscintillators.

One aspect of the present invention includes a scintillator compositionincluding a scintillation compound and a dopant. Scintillation compoundscan include a compound with the formula x₁-x₂-x₃-x₄. In certainembodiments, x₁ can include Cs, x₂ can include Na, x₃ can include La,Gd, or Lu, and x₄ can include a halide. In some embodiments, thescintillator composition can include a single dopant or a mixture ofdopants. Scintillator compositions of the present invention can includea mixture of different compounds (e.g., quaternary compounds) disclosedherein, such as mixed scintillator compositions including two or moredifferent CsXLa halide compounds, where X is Na or Li.

In another aspect, the invention further includes devices, such as aradiation detection device having a scintillator composition including ascintillation compound and a dopant; and a photodetector assemblyoptically coupled to the scintillator composition. The photodetectorassembly can include, for example, a photomultiplier tube, a photodiode, or a PIN detector. The device may further include a dataanalysis, or computer, system for processing and analyzing detectedsignals.

In yet another aspect, the invention includes an X-ray and neutrondetector assembly, including a scintillator composition including ascintillation compound and a dopant, a photodetector assembly, andelectronics or a system for data processing/analysis. For example, thedevice can include electronics configured for performing pulse-shapeanalysis to differentiate gamma ray from neutron emissions.

In yet another aspect, the invention includes a method of performingradiation detection. Such a method can include, for example, providing adetection device having a detector assembly including a scintillatorcomposition including a scintillation compound and a dopant; aphotodetector assembly; and positioning a target within a field of viewof the scintillator as to detect emissions from the target. Emissionscan include, for example, gamma-ray, X-ray, or neutron emissions. Atarget can include various potential sources of detectable emissionsincluding neutron emitters (e.g., plutonium and the like), gamma-raysources (e.g., uranium and the like), X-ray sources, etc. In oneembodiment, for example, the scintillator compositions can be used forimaging applications including medical imaging such as in a method ofperforming PET (e.g., time-of-flight PET) or SPECT. In such anembodiment, the imaging method can comprise injecting or otherwiseadministering a patient with a detectable label, and, after a sufficientperiod of time to allow localization or distribution of the label,placing the patient within the field of view of the detection device.Thus, in some embodiments the target includes a patient or a portion ofa patient's body.

For a fuller understanding of the nature and advantages of the presentinvention, reference should be had to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings. The drawingsrepresent embodiments of the present invention by way of illustration.The invention is capable of modification in various respects withoutdeparting from the invention. Accordingly, the drawings/figures anddescription of these embodiments are illustrative in nature, and notrestrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A through 1D depict ¹³⁷Cs spectra collected with Cs₂NaGdI₆:2% Ce(FIG. 1A), Cs₂NaLaBr₆:0.2% Ce (FIG. 1B), Cs₂NaLaI₆:5% Ce (FIG. 1C), andCs₂NaLuI₆:1% Ce (FIG. 1D) scintillator compositions coupled to a PMT.The energy resolutions of a 662 keV peak for Cs₂NaGdI₆:2% Ce,Cs₂NaLaBr₆:0.2% Ce, Cs₂NaLaI₆:5% Ce, and Cs₂NaLuI₆:1% Ce are 7.6%, 11%,5.25%, and 8.5% (FWHM), respectively. FIGS. 1A, 1C, and 1D furtherdepict ¹³⁷Cs spectra for BGO coupled to a PMT.

FIG. 2A through 2D depict optical emission spectra for Cs₂NaGdI₆:Ce(FIG. 2A), Cs₂NaLaBr₆:Ce (FIG. 2B), Cs₂NaLaI₆:Ce (FIG. 2C), andCs₂NaLuI₆:Ce (FIG. 2D) scintillator compositions upon exposure toX-rays. FIGS. 2A and 2D show optical emission spectra for 1% and 2% Cewith Cs₂NaGdI₆:Ce and Cs₂NaLuI₆:Ce, respectively.

FIG. 3A through 3C depict time profiles for Cs₂NaGdI₆:2% Ce (FIG. 3A),Cs₂NaLaI₆:5% Ce (FIG. 2B), and Cs₂NaLuI₆:1% Ce (FIG. 2C) exposed togamma rays. Risetimes (τ_(r)) for Cs₂NaGdI₆:2% Ce, Cs₂NaLaI₆:5% Ce, andCs₂NaLuI₆:1% Ce were 0.85 ns, 4 ns, and 0.85 ns, respectively, incertain embodiments. Principal decay times (τ_(dl)) for Cs₂NaGdI₆:2% Ce,Cs₂NaLaI₆:5% Ce, and Cs₂NaLuI₆:1% Ce were 55 ns, 50 ns, and 35 ns,respectively.

FIG. 4A through 4C illustrate non-proportionality for Cs₂NaGdI₆:2% Ce(FIG. 4A), Cs₂NaLaI₆:5% Ce (FIG. 4B), and Cs₂NaLuI₆:1% Ce (FIG. 4C)scintillator compositions. The figure shows light output of thescintillator compositions measured under excitation from isotopes suchas ²⁴¹Am (60 keV γ-rays), ⁵⁷Co (122 keV γ-rays), ²²Na (511 keV and 1275keV γ-rays), and ¹³⁷Cs (662 keV γ-rays).

FIG. 5 is a schematic diagram of a detector assembly of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

This invention will be better understood with resort to the followingdefinitions:

A. Rise time, in reference to a scintillation crystal material, shallmean the speed with which its light output grows once a gamma-ray hasbeen stopped in the crystal. The contribution of this characteristic ofa scintillator combined with the decay time contribute to a timingresolution.

B. A Fast timing scintillator (or fast scintillator) typically includesa timing resolution of about 500 ps or less. For certain PETapplications (e.g., time-of-flight (TOF)), the fast scintillator shouldbe capable of localizing an annihilation event as originating fromwithin about a 30 cm distance, i.e., from within a human being scanned.

C. Timing accuracy or resolution, usually defined by the full width halfmaximum (FWHM) of the time of arrival differences from a point source ofannihilation gamma-rays. Because of a number of factors, there is aspread of measured values of times of arrival, even when they are allequal. Usually they distribute along a bell-shaped or Gaussian curve.The FWHM is the width of the curve at a height that is half of the valueof the curve at its peak.

D. Light Output shall mean the number of light photons produced per unitenergy deposited by the detected gamma-ray, typically the number oflight photons/MeV.

E. Stopping power or attenuation shall mean the range of the incomingX-ray or gamma-ray in the scintillation crystal material. Theattenuation length, in this case, is the length of crystal materialneeded to reduce the incoming beam flux to 1/e⁻.

F. Proportionality of response (or linearity). For some applications(such as CT scanning) it is desirable that the light output besubstantially proportional to the deposited energy. For applicationssuch as spectroscopy, non-proportionality of response is an importantparameter. In a typical scintillator, the number of light photonsproduced per MeV of incoming gamma-ray energy is not constant. Rather,it varies with the energy of the stopped gamma-ray. This has twodeleterious effects. The first is that the energy scale is not linear,but it is possible to calibrate for the effect. The second is that itdegrades energy resolution. To see how this occurs, consider ascintillator that produces 300 photons at 150 keV, 160 photons at 100keV and 60 photons at 50 keV. From statistics alone, the energyresolution at 150 keV should be the variability in 300 photons, which is5.8%, or 8.7 keV. If every detected event deposited 150 keV in one stepthis would be the case. On the other hand, if, as it occurs, an eventdeposited 100 keV in a first interaction and then another 50 keV in asecond interaction, the number of photons produced would not be 300 onthe average, but 160+60=220 photons, for a difference of 80 photons or27%. In multiple detections, the peak would broaden well beyond thetheoretical 8.7 keV. The smaller the non-proportionality the smallerthis broadening and the closer the actual energy resolution approachesthe theoretical limit.

The scintillation compositions of the present invention will respond byemitting light after detecting charged particles, high energy photons,and for some embodiments, neutrons, thereby providing usefulscintillation properties. The scintillation compound has the formula,x₁-x₂-x₃-x₄, and can include Cs as x₁, Na as x₂, La, Gd, or Lu as x₃,and a halide such as Br or I as x₄. Gd has a large neutroncross-section. In certain embodiments, a dopant as specified in thespecification and claims, can be added to the scintillator composition.In certain embodiments, the scintillation compound elements exist inatomic ratios of 2:1:1:6 with a dopant, such as Cs₂NaGdI₆:2% Ce.

Scintillator compositions can include a single quaternary compoundselected from those described herein or a mixture of different compoundsdescribed herein. Compositions can, e.g., include mixed scintillatorcompositions with at least two different CsXLa halide compounds, where Xis Na or Li. Thus, mixed scintillator compositions can include a mixtureof at least two quaternary compounds having Na or Li at the secondposition, such that the quaternary compound mixture includes, forexample, Cs(Na,Li)La halide. Halides can include, for example, F, Cl,Br, or I. Certain embodiments can further include mixed scintillatorcompositions that include a mixture of at least two quaternary compoundshaving Na or Li at the second position and at least two differenthalides at the fourth position, such that the quaternary compoundmixture includes, for example, Cs(Na,Li)La(Cl,Br) or any other possiblecombination of halides and Na and Li. Thus, as different compounds mayhave different scintillation characteristics, one advantage of thepresent invention includes the ability to select or customize aparticular mixed composition ratio or formulation, where a particularselection or formulation is at least partially based on intended use ofthe scintillator composition and/or desired properties orscintillation/performance characteristics. For example, a particularmixed quaternary compound composition (e.g., mixed CsXLa halidecomposition such that X is Na or Li) formulation may be selected basedon one or more of a variety of factors, such as desired stopping power(e.g., at least partially as a function of atomic number), neutronresponse, bandgap levels tuning/optimization for Ce emission and lightoutput, pulse shape discrimination, gamma-ray sensitivity, and the like.

The scintillator compositions of the invention are particularly useful,for example, for spectroscopy detection of energetic photons (e.g.,X-rays, gamma-rays), as well as for neutron emission detection. Notablecharacteristics for the scintillation compositions of the inventioninclude surprisingly robust light output, high gamma-ray and neutronstopping efficiency (attenuation), fast response, and goodnon-proportionality. Furthermore, the scintillator compositions can beefficiently and economically produced. Thus, detectors havingscintillator compositions described in the present invention are usefulin a wide variety of applications, including without limitation nuclearand high energy physics research, medical imaging, diffraction,non-destructive testing, nuclear treaty verification and safeguards, andgeological exploration.

The scintillator composition of the present invention can optionallyinclude a “dopant”. Dopants can affect certain properties, such asphysical properties (e.g., brittleness, etc.) as well as scintillationproperties (e.g., luminescence, etc.) of the scintillator composition.The dopant can include, for example, cerium (Ce), praseodymium (Pr),lutetium (Lu), lanthanum (La), europium (Eu), samarium (Sm), strontium(Sr), thallium (Tl), chlorine (Cl), fluorine (F), iodine (I), andmixtures of any of the dopants. The amount of dopant present will dependon various factors, such as the application for which the scintillatorcomposition is being used; the desired scintillation properties (e.g.,emission properties, timing resolution, etc.); and the type of detectiondevice into which the scintillator is being incorporated. For example,the dopant is typically employed at a level in the range of about 0.1%to about 20%, by molar weight. In certain embodiments, the amount ofdopant is in the range of about 0.1% to about 100%, or about 0.1% toabout 5.0%, or about 5.0% to about 20%, by molar weight.

The scintillator compositions of the invention may be prepared inseveral different forms. In some embodiments, the composition is in acrystalline form (e.g., monocrystalline). Scintillation crystals, suchas monocrystalline scintillators, have a greater tendency fortransparency than other forms. Scintillators in crystalline form (e.g.,scintillation crystals) are often useful for high-energy radiationdetectors, e.g., those used for gamma-ray or X-ray detection. However,the composition can include other forms as well, and the selected formmay depend, in part, on the intended end use of the scintillator. Forexample, a scintillator can be in a powder form. It can also be preparedin the form of a ceramic or polycrystalline ceramic. Other forms ofscintillation compositions will be recognized and can include, forexample, glasses, deposits, vapor deposited films, microcolumnar, orother forms suitable for radiation detection as described herein. Itshould also be understood that a scintillator composition might containsmall amounts of impurities. Also, minor amounts of other materials maybe purposefully included in the scintillator compositions to affect theproperties of the scintillator compositions.

Methods for making crystal materials can include those methods describedherein and may further include other techniques. Typically, theappropriate reactants are melted at a temperature sufficient to form acongruent, molten composition. The melting temperature will depend onthe identity of the reactants themselves (see, e.g., melting points ofreactants), but is usually in the range of about 300° C. to about 1350°C. Non-limiting examples of the crystal-growing methods can includecertain techniques of the Bridgman-Stockbarger methods; the Czochralskimethods, the zone-melting methods (or “floating zone” method), thevertical gradient freeze (VGF) methods, and the temperature gradientmethods. See, e.g., Example 1 infra. (see also, e.g., “LuminescentMaterials”, by G. Blasse et al, Springer-Verlag (1994) and “CrystalGrowth Processes”, by J. C. Brice, Blackie & Son Ltd (1986)).

In the practice of the present invention, attention is paid to thephysical properties of the scintillator material. In particularapplications, properties such as hygroscopy (tendency to absorb water),brittleness (tendency to crack), and crumbliness should be minimal.

TABLE I Properties of Scintillators Wave- Light length Principal OutputOf Rise- Decay (Photons/ Density Emission time Time Material MeV)(g/cm³) (nm) (ns) (ns) NaI(Tl) 38,000 3.67 415 >10 230 CsI(Tl) 52,0004.51 540 >10 1000 LSO 24,000 7.4 420 <1 40 BGO  8,200 7.13 505 >1 300BaF₂ 10,000 4.88 310, slow <0.1 620, slow ~2,000 220, fast 0.6, fastfast GSO  7,600 6.7 430 ~8 60 CdWO₄ 15,000 8.0 480 5000 YAP 20,000 5.55370 <1 26 Cs₂NaLaBr₆:Ce 12,000 3.91 386 30 55 Cs₂NaGdI₆ Ce 26,000 ~4 4310.85 55 Cs₂NaLaI₆:Ce 49,000 ~4 429 4 50 Cs₂NaLuI₆:Ce 27,000 4.6 428 0.8535

Table I provides a listing of certain properties of a number ofscintillators. Compared to other commercially available scintillators,including CsI, which is among the scintillation materials with thehighest known light output, the scintillator compositions of theseinventions produce comparable light output. In addition, they have afast principal decay-time constant.

As set forth above, scintillator compositions of the present inventionmay find use in a wide variety of applications. In one embodiment, forexample, the invention is directed to a method for detecting energyradiation (e.g., gamma-rays, X-rays, neutron emissions, and the like)with a scintillation detector including the scintillation composition ofthe invention.

FIG. 5 is a schematic diagram of a detector assembly of the presentinvention. The detector 10 includes a scintillator 12 optically coupledto a light photodetector 14 or imaging device. The detector assembly 10can include a data analysis, or computer, system 16 to processinformation from the scintillator 12 and light photodetector 14. In use,the detector 10 detects energetic radiation emitted form a source 18.

A data analysis, or computer, system thereof can include, for example, amodule or system to process information (e.g., radiation detectioninformation) from the detector/photodetectors can also be included in aninvention assembly and can include, for example, a wide variety ofproprietary or commercially available computers, electronics, or systemshaving one or more processing structures, a personal computer,mainframe, or the like, with such systems often comprising dataprocessing hardware and/or software configured to implement any one (orcombination of) the method steps described herein. Any software willtypically comprise machine readable code of programming instructionsembodied in a tangible media such as a memory, a digital or opticalrecording media, optical, electrical, or wireless telemetry signals, orthe like, and one or more of these structures may also be used totransmit data and information between components of the system in any ofa wide variety of distributed or centralized signal processingarchitectures.

The detector assembly typically includes material formed from thescintillator composition described herein (e.g., one or morescintillator crystals). The detector further can include, for example, alight detection assembly including one or more photodetectors.Non-limiting examples of photodetectors include photomultiplier tubes(PMT), photodiodes, CCD sensors, image intensifiers, and the like.Choice of a particular photodetector will depend in part on the type ofradiation detector being fabricated and on its intended use of thedevice. In certain embodiments, the photodetector may beposition-sensitive.

The detector assemblies themselves, which can include the scintillatorand the photodetector assembly, can be connected to a variety of toolsand devices, as mentioned previously. Non-limiting examples includenuclear weapons monitoring and detection devices, well-logging tools,and imaging devices, such as nuclear medicine devices (e.g., PET).Various technologies for operably coupling or integrating a radiationdetector assembly containing a scintillator to a detection device can beutilized in the present invention, including various known techniques.

The detectors may also be connected to a visualization interface,imaging equipment, or digital imaging equipment (e.g., pixilated flatpanel devices). In some embodiments, the scintillator may serve as acomponent of a screen scintillator. For example, powdered scintillatormaterial could be formed into a relatively flat plate, which is attachedto a film, such as photographic film. Energetic radiation, e.g., X-rays,gamma-rays, neutron, originating from a source, would interact with thescintillator and be converted into light photons, which are visualizedin the developed film. The film can be replaced by amorphous siliconposition-sensitive photodetectors or other position-sensitive detectors,such as avalanche diodes and the like.

Imaging devices, including medical imaging equipment, such as the PETand SPECT devices, and the like, represent another important applicationfor invention scintillator compositions and radiation detectors.Furthermore, geological exploration devices, such as well-loggingdevices, were mentioned previously and represent an importantapplication for these radiation detectors. The assembly containing thescintillator usually includes, for example, an optical window at one endof the enclosure-casing. The window permits radiation-inducedscintillation light to pass out of the scintillator assembly formeasurement by the photon detection assembly or light-sensing device(e.g., photomultiplier tube, etc.), which is coupled to the scintillatorassembly. The light-sensing device converts the light photons emittedfrom the scintillator into electrical pulses that may be shaped anddigitized, for example, by the associated electronics. By this generalprocess, gamma-rays can be detected, which in turn provides an analysisof geological formations, such as rock strata surrounding the drillingbore holes.

In many of the applications of a scintillator composition as set forthabove (e.g., nuclear weapons monitoring and detection, imaging, andwell-logging and PET technologies), certain characteristics of thescintillator are desirable, including high light output, fast rise timeand short decay time, good timing resolution, and suitable physicalproperties. The present invention is expected to provide scintillatormaterials that can provide the desired high light output and initialphoton intensity characteristics for demanding applications of thetechnologies. Moreover, the invention scintillator compositions are alsoexpected to simultaneously exhibit the other important properties notedabove, e.g., short decay time and good stopping power. Furthermore, thescintillator materials are also expected to be produced efficiently andeconomically, and also expected to be employed in a variety of otherdevices which require radiation/signal detection (e.g., gamma-ray,X-ray, neutron emissions, and the like).

The following examples are intended to illustrate but not limit theinvention.

EXAMPLES Example 1

The present example provides a method for growing and providescharacterization for the scintillator composition crystals. Thefollowing examples are offered by way of illustration, not by way oflimitation.

Crystal Growth of Compounds Exemplified with Cs₂NaLaBr₆, Cs₂NaGdI₆,Cs₂NaLaI₆, and Cs₂NaLuI₆

In one example, a one zone Bridgman furnace was used for crystal growth.Typical growth rates for the Bridgman process are about 1-6 mm/hour.Growth rates ranging from about 1 mm/day to about 1 cm/hour may beutilized. The range of rates may be extended to improve materialquality.

Cs₂NaLaBr₆, Cs₂NaGdI₆, Cs₂NaLaI₆, and Cs₂NaLuI₆ have a cubic crystalstructure. The densities of Cs₂NaLaBr₆, Cs₂NaGdI₆, Cs₂NaLaI₆, andCs₂NaLuI₆ are between about 3.9 and about 4.6 g/cm³. The compositionsmelt congruently at approximately 78, 925, 778, and 1050° C.,respectively, and therefore their crystals can be grown using melt basedmethods such as those described by Bridgman and Czochralski. Thesemelt-based processes are well suited for growth of large volume crystals(Brice, Crystal Growth Processes, Blackie Halsted Press (1986)). TheBridgman method has been used for growing Cs₂NaLaBr₆, Cs₂NaGdI₆,Cs₂NaLaI₆, and Cs₂NaLuI₆. Both the vertical and horizontal orientationsof the Bridgman method can be used in producing crystals of the presentinvention. In certain embodiments, the vertical Bridgman method was usedin producing crystals examined and discussed below.

Cs₂NaLaBr₆: Single crystals of this material were grown by the Bridgmantechnique in vertical silica ampoules under vacuum. Starting materialswere CsBr (Aldrich, anhydrous, 99.9%), NaBr (Aldrich, anhydrous, 99.9%),and LaBr₃ (Aldrich, anhydrous, 99.99+%).

Cs₂NaGdI₆: Single crystals of this material were grown by the Bridgmantechnique in vertical silica ampoules under vacuum. Starting materialswere CsI (Aldrich, anhydrous, 99.9%), NaI (Aldrich, anhydrous, 99.9%),and GdI₃ (Aldrich, anhydrous, 99.99+%).

Cs₂NaLaI₆: Single crystals of this material were grown by the Bridgmantechnique in vertical silica ampoules under vacuum. Starting materialswere CsI (Aldrich, anhydrous, 99.9%), NaI (Aldrich, anhydrous, 99.9%),and LaI₃ (Aldrich, anhydrous, 99.99+%).

Cs₂NaLuI₆: Single crystals of this material were grown by the Bridgmantechnique in vertical silica ampoules under vacuum. Starting materialswere CsI (Aldrich, anhydrous, 99.9%), NaI (Aldrich, anhydrous, 99.9%),and LuI₃ (Aldrich, anhydrous, 99.99+%).

Scintillation Properties of Scintillator Compositions

Scintillation properties of small Bridgman grown scintillationcomposition crystals (≦300 mm³) have been characterized. Thisinvestigation involved measurement of the light output, the emissionspectrum, and the scintillation decay time of the crystals. Energyresolution of sample crystals and their proportionality of response werealso measured.

1. Light Output and Energy Resolution

As shown in FIGS. 1A, B, C, and D, the energy resolution of the 662 keVphotopeak recorded with the scintillator compositions has been measuredto be in the vicinity of 7.6%, 11%, 5.25%, and 8.5% (FWHM) at roomtemperature for Cs₂NaGdI₆:2% Ce, Cs₂NaLaBr₆:0.2% Ce, Cs₂NaLaI₆:5% Ce,and Cs₂NaLuI₆:1% Ce, respectively. The light output of scintillatorcomposition crystals was measured by comparing their response to 662 keVγ-rays (¹³⁷Cs source) to the response of a BGO scintillator to the sameisotope (see FIGS. 1A, C, and D). This measurement involved opticalcoupling of a scintillator crystal to a photomultiplier tube (withmulti-alkali S-20 photocathode), irradiating the scintillator with 662keV photons, and recording the resulting pulse height spectrum. In orderto maximize light collection, the scintillator composition crystal waswrapped in reflective white Teflon tape on all faces (except the onecoupled to the PMT). An index matching silicone fluid was also used atthe PMT-scintillator interface. A pulse height spectrum was recordedwith a scintillator composition crystal. This experiment was thenrepeated with a BGO scintillator. Comparison of the photopeak positionobtained with the scintillator composition for 662 keV photon energy tothat with BGO provided estimation of light output for the scintillatorcomposition crystal. FIGS. 1 A, B, C, and D show the pulse heightspectra for a scintillator composition under ¹³⁷Cs irradiation andamplifier shaping time of 4.0 μs. This shaping time is long enough toallow full light collection from both the scintillators. The PMT biasand amplifier gain were the same for both spectra. Based on the recordedphotopeak positions for each scintillator composition and BGO, lightoutput of Cs₂NaGdI₆:2% Ce, Cs₂NaLaBr₆:0.2% Ce, Cs₂NaLaI₆:5% Ce, andCs₂NaLuI₆:1% Ce crystals was estimated to be about 26,500 photons/MeV,12,000 photons/MeV, 48,500 photons/MeV, and 27,000 photons/MeV,respectively.

2. Emission Spectrum

Normalized emission spectra for the scintillator compositions are shownin FIGS. 2A through 2D. The scintillator composition samples wereexcited with radiation from a Philips X-ray tube having a Cu target,with power settings of 40 kVp and 20 mA. The scintillation light waspassed through a McPherson monochromator and detected by aphotomultiplier tube. The peak emission wavelength for the Cs₂NaGdI₆:Ce,Cs₂NaLaBr₆:Ce, Cs₂NaLaI₆:Ce, and Cs₂NaLuI₆:Ce samples was atapproximately 431 nm, 386 nm, 429 nm, and 428 nm, respectively. Peakemission wavelengths in this range are attractive for gamma-rayspectroscopy because they match well with the spectral response of thephotomultiplier tubes as well as a new generation of siliconphotodiodes.

3. Time Profiles

FIGS. 3 A, B, and C show the time profiles recorded for Cs₂NaGdI₆:2% Ce,Cs₂NaLaI₆:5% Ce, and Cs₂NaLuI₆:1% Ce samples, respectively. Timeprofiles of the scintillator compositions have been measured under gammaray excitation using the delayed coincidence method (Bollinger andThomas, Rev. Sci. Instr. 32:1044 (1961)). For Cs₂NaGdI₆:2% Ce, the risetime of the scintillation pulse is ˜0.85 ns and the principal decay timeis about 55 ns. For Cs₂NaLaI₆:5% Ce, the rise time of the scintillationpulse is ˜4 ns and the principal decay time is about 50 ns. ForCs₂NaLuI₆:1% Ce, the rise time of the scintillation pulse is ˜0.85 nsand the principal decay time is about 35 ns. FIGS. 3A-C also showsecondary, slower decay components present in time profiles.

4. Non-Proportionality

As shown in FIGS. 4 A, B, and C, the non-proportionality of Cs₂NaGdI₆:2%Ce, Cs₂NaLaI₆:5% Ce, and Cs₂NaLuI₆:1% Ce scintillator compositions wasevaluated, respectively. Non-proportionality (as a function of energy)in light yield can be one of the important reasons for degradation inenergy resolution of established scintillators such as NaI(Tl) andCsI(Tl) (Dorenbos et al., IEEE Trans. Nuc. Sci. 42:2190 (1995)). Lightoutput of the scintillator compositions was measured under excitationfrom isotopes such as ²⁴¹Am (60 keV γ-rays), ⁵⁷Co (122 keV, 136 keV, and14.4 keV γ-rays), ²²Na (511 keV and 1275 keV γ-rays) and ¹³⁷Cs (662 keVy-rays). The test crystals were wrapped in Teflon tape and coupled to aPMT. Pulse height measurements were performed using standard NIMequipment with the scintillator exposed to different radioisotopes. Thesame settings were used for the PMT and pulse processing electronics foreach isotope. From the measured peak position and the known y-ray energyfor each isotope, the light output (in photons/MeV) at each y-ray energywas estimated. The data points were then normalized with respect to thelight output value at 662 keV energy and the results (shown in FIGS. 4A, B, and C) indicated that Cs₂NaGdI₆:2% Ce, Cs₂NaLaI₆:5% Ce, andCs₂NaLuI₆:1% Ce were very proportional scintillators. Over in the energyrange from about 60 to about 1275 keV, the non-proportionality in lightyield was less than about 5%, typically between about 2 to 3% (forcorresponding values for other established scintillators see, e.g.,Guillot-Noel et al., IEEE Trans. Nuc. Sci 46: 1274-1284 (1999)).

Overall, these measurements indicated that the scintillator compositionsas described in the present invention have high light output, fastresponse and show good qualities in terms of light output, energyresolution, speed and exceptional non-proportionality.

Although the invention has been described with reference to the aboveexamples, it will be understood that modifications and variations areencompassed within the spirit and scope of the invention. Accordingly,the invention is limited only by the following claims along with theirfull scope of equivalents. All publications and patent documents citedin this application are incorporated by reference in their entirety forall purposes to the same extent as if each individual publication orpatent document were so individually denoted.

1. A mixed scintillator composition, comprising at least two differentCsXLa halide compounds and a dopant, wherein X is Na or Li.
 2. Thescintillator of claim 1, wherein the different CsXLa halide compoundsare selected from Cs₂NaLaF₆, Cs₂NaLaCl₆, Cs₂NaLaBr₆, Cs₂NaLaI₆,Cs₂LiLaF₆, Cs₂LiLaCl₆, Cs₂LiLaBr₆, and Cs₂LiLaI₆.
 3. The scintillator ofclaim 1, wherein the dopant comprises Ce.
 4. The scintillator of claim1, wherein the dopant is present at less than about 20% by molar weight.5. The scintillator composition of claim 1, wherein the dopant comprisesCe present at equal to or less than about 5% by molar weight.
 6. Thescintillator of claim 1, wherein the scintillator comprises acrystalline, ceramic, or polycrystalline ceramic form.
 7. A radiationdetection device, comprising a mixed scintillator composition comprisingat least two different CsXLa halide compounds and a dopant, wherein X isNa or Li; and a photodetector assembly optically coupled to thescintillator.
 8. The device of claim 7, wherein the photodetectorassembly comprises a photomultiplier tube, a photodiode, a PIN detector,charge-coupled device, or an avalanche detector.
 9. The device of claim7, further comprising a radiation source.
 10. The device of claim 7,further comprising a computer system coupled to the photodetectorassembly so that the computer outputs image data in response to detectedradiation.
 11. The device of claim 10, wherein the computer comprisesinstructions for constructing an image from detected radiation.
 12. Thedevice of claim 7, wherein the device is configured to detect x-rays,gamma-rays, or a combination thereof.
 13. The device of claim 7, whereinthe device comprises a scintillator composition comprising Li and thedevice is configured to detect x-rays, gamma-rays, neutrons, or anycombination thereof.
 14. A method of performing radiation detection,comprising: providing a detection device comprising a mixed scintillatorcomposition comprising at least two different CsXLa halide compounds anda dopant, wherein X is Na or Li; and a photodetector assembly opticallycoupled to the scintillator; and positioning the device such that aradiation source is within a field of view of the scintillatorcomposition so as to detect emissions from the source.
 15. The method ofclaim 14, wherein the emissions comprise gamma-ray or X-ray emissions.16. The method of claim 14, wherein the scintillator compositioncomprises Li and the emissions comprise neutron emissions.
 17. Themethod of claim 14, wherein a patient is positioned between theradiation source and the scintillator composition.
 18. The method ofclaim 14, wherein the radiation source comprises a patient.